[C++] Add casting option to set unsafe casts to null rather than some garbage value

[asfimport]

Null is the obvious choice when certain casts fail, like string to number, but in other kinds of unsafe casts there may be more ambiguity.

Reporter: Wes McKinney / @wesm

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)
• [C++] Type casting function kernel suite (Is contained by)

_{Note: This issue was originally created as ARROW-1489. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm: This might yield some code bloat but would still be useful to have in our repertoire

[asfimport]

David Li / @lidavidm: Additionally it seems right now, even with unsafe cast enabled, a failed string-to-float cast always raises an ArrowInvalid (it may make sense to have an option to allow filling a null or nan or some other value instead).

[asfimport]

Wes McKinney / @wesm: Right, we might start with just the string-to-float "problem" — it probably just means generating some array functions (with new names – e.g. more specifically, like "string_to_double") that do the errors-as-null behavior.